1. Field of the Invention
The invention relates to compositions comprising organosilicon compounds having different viscosities, to processes for preparing them, and to the use thereof as defoamers.
2. Description of the Related Art
In many liquid systems, more particularly in aqueous systems, which include surface-active compounds as desired or else unwanted constituents, it is possible for problems to occur as a result of foaming if these systems come into more or less intensive contact with gaseous substances, such as during the gassing of wastewaters, during the intensive stirring of liquids, during distillation, washing or coloring operations, or during dispensing processes, for example.
This foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have proven particularly appropriate. Defoamers based on siloxanes are prepared according to U.S. Pat. No. 3,383,327 A, for example, by heating hydrophilic silica in polydimethylsiloxanes. Using basic catalysts allows the effectiveness of such defoamers to be improved, as disclosed in U.S. Pat. No. 3,560,401 A. An alternative is to disperse hydrophobized silica in a polydimethylsiloxane, in accordance with DE 29 25 722 A1, for example.
Nevertheless, the effectiveness of the resulting defoamers is usually in need of improvement. Thus U.S. Pat. No. 4,145,308, for example, describes a defoamer preparation which as well as a polydiorganosiloxane and silica also comprises a copolymer made up of (CH3)3SiO1/2 and SiO2 units.
Copolymers made up of (CH3)3SiO1/2 and SiO2 units are said to be advantageous also in combination with siloxanes which carry terminal long alkyl groups, as described in EP-A-301 531 (corresponding U.S. Pat. No. 4,919,843 A). In these formulations, mixtures of fumed and precipitated silicas are used as well.
EP-B-726 086 achieves enhanced effectiveness for the defoamer formulation by using a mixture of pretreated hydrophobized silica and silica hydrophobized in situ.
The use of partly crosslinked, in some cases already rubberlike polydimethylsiloxanes is said to contribute to an increase in defoamer effect. In this regard, reference may be made, for example, to U.S. Pat. No. 2,632,736, EP-B 163 541, EP-B 217 501, EP-A 273 448 and EP-A 434 060. These products, though, are generally of very high viscosity and are difficult to handle or to process further.
Alternatively to partially crosslinked siloxanes, linear polydimethylsiloxanes of high viscosity are also used in defoamer formulations. According to the teaching of U.S. Pat. No. 4,395,352, a marked improvement in defoamer effect is achieved simply by using polydimethylsiloxanes having a viscosity of up to 30,000 mm2/s in the formulations.
EP-B 163 398 uses oils up to a viscosity of up to 200,000 mm2/s for enhancing the defoamer effect in mixtures with oils at 100-5000 mm2/s, silicone resin, and silica. In this case the defoamer effect can be improved simply by admixing the high-viscosity siloxane subsequently.
According to EP 1 750 524 B1, the defoaming effect is made better, in contrast, if an emulsion with a defoamer based on a diorganosiloxane having a viscosity of 1000-10,000 mPas is admixed with a diorganosiloxane having a viscosity of 100-1050 mPas.
In strongly foaming, surfactant-rich systems, however, the known defoamer formulations do not always have a sufficiently long-lasting effectiveness and compatibility, or else, owing to the high viscosity on the basis of the degree of branching or degree of crosslinking attained, they are difficult to handle.